J. Phys. Chem. 1980, 84, 821-826
indicate that the three parameters are not strongly correlated. The quality of the fit does not change significantly unless one (or more) of the three parameters is fixed at a value more than two standard deviations removed from the best-fit value. The value of V,, the effective optical absorption cross section of OH, can be shown to be consistent with earlier measurements in this laboratory on the OH radical in pulse-irradiated water vapor.16 Despite the complexity of the kinetic model, we believe that the value of kl obtained is reasonably accurate and supports the "high" values of kl previously mentioned. Most of the reactions in Table I are of relatively minor importance, and errors in their rate constants do not markedly affect kl. The most significant rate constant, other than h,, in terms of its effect on the fit is h3. A value of 0.5 X lo-"' for h3would be expected on the basis of the value k3 with N2 as t2 third body18 if we use the reportedTg relative third-body efficiencies for N2,Ar, B20,and 02. However, WI: find that a value of h3 N 0.8 X cm3 molecule-l results in a better fit. Making k3 even larger causes the H02decay to be too slow over the first 20 ks. Hence, under the conditions of these experiments a h3 value of (0.8 f 0.2) X 1O-I' cm3 molecule-l s-l is indicated. The effect of'the value used for h3 on the "best-fit" value of kl was not large, however. The change in kl over the was less than 5%. range h3 = 0.5 X 10-11-1.1 X Because of the known effect of [H20] on the value of k6,6~8~20 it is reasonable to ask whether the presence of 3.9 X loT7molecules of H 2 0 has an effect on the value obtained for hl. Therefore, the H02decay was examined ~ , X loT7molecules of a t 3.9 X loT9molecules of Ar ~ m - 2.6 O2~ m - and ~ , concentrations of H20from 6 X 1016-5 X lo1' molecules CI+. No evidence of a significant change in the H02 decay rate was observed, indicating that reaction 1
821
is apparently not appreciably affected by the complexing of H02by H20. The fact that hl is about 20 times greater than k6 is the reason that the known variation of h, with [H20]does not appreciably influence the observed H02 decay in the latter series of experiments. Achnowledgment. We thank Dr. C. Jonah for the least-squares fitting program. References and Notes (1) Work performed under the auspices of the Office of Basic Energy Sciences of the U S . Department of Energy. (2) Department of Chemistry, Malcolm X College, Chicago, IL 60612. (3) Scientific Research Laboratory, Ford Motor Company, Dearborn, MI 48121. (4) J. S.Chang and F. Kaufman, J . Phys. Chem., 82, 1683 (1979). (5) W. Hack, A. W. Preuss, and H. Gg.Wagner, Ber. Bunsenges. Phys. Chem., 82, 1'167 (1978). (6) W. 8. DeMore, J. Phys. Chem., 83, 1113 (1979). (7) S. Gordon, W. LAulac, and P. Nangia, J. phys. Chem., 75, 2087 (1971). ( 8 ) E. J. Hamilton, Jr., and R. R. Lii, Int. J . Chem. Kinet., 9, 875 (1977). (9) R. R. Lii, R. A. Gorse, Jr., M. C. Sauer, Jr., and S.Gordon, J . Phys. Chem., companion paper in this issue. 10) R. F. Hampson, Jr., and D. Garvin, Eds., Nat. Bur. Stand. (U.S.), Spec. Publ., No. 513 (1978). 11) A. R. Ravishankara, P. H. Wine, and A. 0. Langford, J . Chem. Phys., 70, 984 (1979). 12) R. R. Lii, M. C. Sauer, Jr., and S.Gordon, J. Phys. Chem., preceding paper in this issue. 13) T. T. Paukert and H. S.Johnston, J. Chem. Phys., 56, 2824 (1972). 14) M. Griggs, J . Chem. Phys., 49, 857 (1968). 15) J. G. Caivert and J. N. Pitts, Jr., "Photochemistry", Wiley, New York 1966. (16) S.Gordon andl W. A. Mulac, Int. J . Chem. Kinet., Symp., No. 1, 289-299 (1975). (17) P. Kebarle in "Ion-Molecule Reactions", J. Franklin Ed., Plenum Press, New York, 1972, Chapter 7, pp 328-332. (18) D. W. Trainor and C. W. von Rosenberg, Jr., J . Chem. Phys., 61, 1010 (1974). (19) G. Black and G. Porter, Proc. R . SOC.London, Ser. A , 266, 185 (1962). (20) R. A. Cox and J. P. Burrows, J. Phys. Chem., 83, 2560 (1979).
Production of OH from Photolysis of HOC1 at 307-309 nm Mario J. Molina,*st Takashi Ishiwata, and Luisa 1. Molina Department of Chemistry, University of California, Irvine, California 927 17 (Received September 7, 1979) Publication costs assisted by the National Aeronautics and Space Administration
Laser-induced fluorescence has been used to measure the amount, of OH produced in the laser photolysis of HOCl at -310 nm. The HOCl was generated by the C120 + H20 and by the HC1+ ClzO reactions. The laser technique samples OH on a microsecond time scale so that secondary radical reactions cannot contribute significantly to the observed signals. The fluorescence signals were calibrated with 03-H20 mixtures, the reactions O3 f hu O(lD) + O2and O('D) + H20 20H yielding known amounts of OH. The value for the absorption cross section of HOCl at 310 nm has been determined to be -6 X cm2,assuming K,, = 0.082 for the ClzO + H20 ~ ' 2HOC1 t system and assuming unit quantum yield for the production of OH. The present investigation further supports earlier experimental work concluding that HOCl will photodissociate rapidly and will not be an important holding tank for chlorine in the stratosphere. These results are in disagreement with theoretical calculations of the 1JV absorption spectrum of HOCl in the 300-350-nm wavelength region.
-
-
Introduction The ultraviolet spectrum of HOCl has been the subject of several recent investigations because of the potential role that this species may play in stratospheric chemistry. The UV spectrum is needed to estimate its rate of phoDreyfus Teacher-Scholar.
0022-3654/80/2084-0821$01.00/0
todecomposition in the stratosphere where HOCl is formed by the rapid reaction between HOz and C10 radicals.' The theoretical calculations of Jaffe and LanghoffZ and some experiments done by Timmons' and by Hisatsune3indicate negligible HOCl absorption at wavelengths longer than 300 nm; in contrast,, our earlier work4 as well as the experimental results of Jaffe and DeMore5 and of Knauth et a1.6 0 1980 American
Chemical Society
822
,,
The Journal of Physical Chemistry, Vo/. 84, No. 8, 1980
' ~
B R E W S T E R ANGLE WINDOW
,/
LT R I A G P& Y r s
PHOTOrr:
-
-
MULTIPLIER
Molina et al. 5
1
r
I
I
I
I
40
60
80
I
I
h
1
PHOTODIODE
AMPLIFIERS
TUNABLE
LASER
TRIGGER CONTROL
SIGNAL AVERAGE R
COMPUTER
LEI TERMINAL
Figure 1. Schematic of the laser photolysis-laser-induced fluorescence apparatus.
indicates that absorption beyond 300 nm is strong enough to make HOCl unimportant as an inert reservoir for stratospheric ~ h l o r i n e . ~ All previous investigations of the UV spectrum of HOCl have consisted of measurements of UV absorptivities of equilibrium C120-H20-HOC1 mixtures. In the present work we have studied the HOCl photodissociation rate around 310 nm by measuring directly OH concentrations produced by photolysis of HOCl on a sufficiently short time scale to preclude the occurrence of secondary reactions. A microsecond laser pulse functioned both as a photolytic source and as a fluorescence excitation source: at 307-309 nm, the laser samples ground-state, thermalized OH radicals (u = 0, K = 2-7).s The OH fluorescence signals were calibrated against signals from 03-H20-He mixtures photolyzed under identical conditions, the OH being generated by the rapid O(lD) + HzO reaction.
Experimental Section A schematic diagram of the apparatus is shown in Figure 1. The frequency-doubled, flash-lamp-pumped dye laser (Chromatix CMX-4) had an average power of -0.4 mJ/ pulse and a bandwidth of 0.05 nm (except as noted below). The laser frequency was monitored with a 0.45-m monochromator (McKee-Pedersen 1018B), and a Corning CS 7-54 filter was used to block the fundamental laser frequency. The fluorescence signals were detected by an EM1 9782 QA photomultiplier through a 20-nm bandpass interference filter centered at 307 nm. The fluorescence signal (F)and the signal from a reference photodiode ( R ) were processed by a dual-channel boxcar integrator (PAR 162/164) and subsequently digitized and stored by a signal averager computer system (Inotech Ultima II/Data General Nova 3). The duration of the laser pulse and of the fluorescence signal was about 1.2 ps. The laser was operated a t 10 Hz, signals were collected for -2 s in each run, and the relative integrated emission intensity (S) was calculated by averaging the F / R ratios. Intensity values decreased by less than 5% between consecutive runs while keeping the same sample in the fluorescence cell. All measurements reported below were carried out with fresh samples a t 298 K; the emission intensities were measured
*
'3
095~10-~
v)
0
20
[031 , m t o r r
100
Flgure 2. Fluorescence intensity signals from photolysis of 0,-H,O-He mixtures as a function of ozone concentration.
with good reproducibility at least three times for each different sample composition. The HOC1 samples were prepared by mixing Cl2O,H20, and He in 3-L bulbs, allowing several hours for equilibration. For some experiments HOCl was generated by mixing HC1 and C120 directly in the fluorescence cell. Samples containing 03,H,O, and He were premixed in 3-L bulbs, and the ozone concentration was established by UV spectroscopy. ClzOwas prepared and handled as described in our earlier p~blication.~The ozone generated by a commercial ozonizer was collected on silica gel a t 195 K and purified by bulb-to-bulb distillation prior to each experiment. The He, Nz, HCI, C02, Oz, and CH4 were research grade gases used without further purification. The samples were handled in a conventional greaseless vacuum line, and the pressures were monitored with MKS capacitance manometers.
Results High-Resolution Fluorescence Emission Spectrum. The dye laser was operated with an etalon inside the cavity to provide narrow bandwidth pulses (Ax N 0.003 nm), and the fluorescence signals were recorded while scanning the laser wavelength around 310 nm. The emission spectra from 03-H20 as well as from C120-H20-HOC1 mixtures were identical with the well-characterized OH ( A 2 P X2n)spe~trum.~ Emission f r o m Pure Reactants. Signals from each of the pure reactants were monitored. As expected, at a pressure of -1 torr, pure 03,C120, H20, or HC1 did not emit; neither did He at pressures up to -200 torr. The ClzO samples gave rise to OH signals unless the water adsorbed on the glass surfaces was previously removed by conditioning the fluorescence cell. For gases such as Nz, CHI, C02,and O2 a t pressures in the 50-100-torr range a broad emission was observed,that is, emission uncorrelated with the OH spectrum, presumably arising from Rayleigh scattering of the laser light. Some broad emission was also observed from H 2 0 samples at 6-10 torr and from He samples above 200 torr; however, in no case did this background emission amount to more than 5% of the OH fluorescence signals discussed below. Emission from the HCl + ClzO Reaction Products. Water-free HOCl samples were prepared by rapidly mixing 0.01-0.9 torr of HCI with 0.03-0.5 torr of ClzO directly in the fluorescence cell. OH signals could be detected immediately after mixing and reached a maximum in less than 1 min. Emission from 03-H20-He Mixtures. The OH signal was linearly proportional to [Os] when [HZO] was kept constant (Figure 2). Increasing [HzO]while keeping LO31 constant first resulted in an increase, and eventually in a
-
Production of O H from Photolysis of HOC1
The Journal
TOTAL P R E S S U R E , l o r r 200 300
a00
or' Physical Chemistry, Vol. 84, No. 8, 1980 823
TABLE I: Observed Fluorescence Intensity Signals in the Photolysis of HOICl-C1,O-H,O Mixtures Relative to Signals from 0,-H,O Mixtures, Both with [H,O] = 1 torr ~~
* Iv,
z
2.0-
'
1 2 3
I-
15r
0
4 5 6 7 8 9 10 11 12 13
1 . 1 x 10-2
4.5x 1 0 - 2 W
a
i
l 2
0
6
4
Flgure 3. Fluorcsscence intensity signals from photolysis of 03-H,0-He mixtures as a fiunction of water concentration, normalized to unity at [H,O] = 1 torr. The solid curve is the computer-simulated signal. TOTAL P R E S S U R E , t o r r 300
200
IOU
400
500
5 -
-
a
= 3-
2 -
J
L
1.4 1.25 0.89 1.07 1.22 1.03 1.22 0.81 0.68 0.68 0.33 0.36 0.26
2.9 2.0 2.0 1.8 0.94 0.94 0.51
1.5 1.0 0.58
[O,l,
lo-'
torr -.
5.2 5.2 5.2 2.0 1.9 2.1 1.9 1.9 2.1 2.0 2.1 1.9 1.9
reaction
ha
O ( ' D ) t H,O 2 0 H 2.0 X lo-'' 2.3 X 10.'' O('D) + 0, 2 0 , OH? i- H,O OH t H,O 6 X lo-'' OH* OH[ + hv 1.25 X lo6 OH* + H,O -+ OH t H,O 4.5 X 10.'' Units: cm3 miolecule-' s - ' , except R6, s - ' .
R2: R3: R4: R6: R7:
-+
-+
-+
--f
ref 18 18 this work 19 10
Discussion OH from Photolysis of O,-H,O Mixtures. The behavior of the fluorescence signals reported above may be explained by the following rather simplified scheme:
L
0
30 20 10 4.5 3.8 3.1
309 nm
= 1torr) measured relative to signals from 03-Hz0 mixtures (also with [HzO] = 1torr). An 03-H20 calibration signal was recorded before and after each HOC1 r m .
E
2
308 nm
TABLE 11: Rate Constants Employed in the Computer Simulation of the Laser Photolysis of 0,-H,O Mixtures
a
.-:4
l o - * torr
8
[ H z O I , torr
k .-0c
[C401",
. run
W
relative signal
2
4
I
.
6 H20 1 , t o r r
S
2
10
Flgure 4. Fluorescence intensity signals from photolysis of HOCICIzO-H20 mixtures as a function of water concentration for samples with constant [ HOCI] / [H,O] .
decrease, in the emission signals. Increasing [H20] and [O,] in the same proportion gave increasing signals which leveled off to a value dependent only on the [03]/[H20] ratio. As shown in Figure 3, the signal increased by a factor of 2.2 f 0.2 when [H20] was raised from 1to 10 torr (with the corresponding increase in [O,] of a factor of 10) regardless of the [03]/[HzO] ratio. For samples of fixed composition the fluorescence emission intensity was proportional to the square of the laser power for powers in the range 0.04-0.4 mJ/pulse. For routine calibration purposes we employed samples with [H20] = 1 torr. (Significantly longer times were required to pump and condition the vacuum line and fluorescence cell when [H20] k 5 torr.) Emission from ClzO-H20-HOC1- He Mixtures. Figure 4 shows that the emission intensity values obtained from mixtures prelpared with a given [C120]o/[H20]oratio were nearly independent of [H20],(the subscript refers to the initial concentrations, Le., before equilibration). The signals increased nonlinearly with [C120]ofor samples with fixed [H,O],. Here again, the fluorescence signals were proportional to the square of the laser power over the power range under investigation. Table I lists the observed fluorescence intensity signals from C120-H20-HOC1 equilibrium mixtures (with [ H20],
O3 + hu
-
O(lD) + O2
O(lD) + H 2 0
- +
O(lD) + O3 OIHt
+ H,O OH
OH*
-+
+ H20
20Ht
(R2)
202
033)
OH
+ hu
OH*
-
-+
-
H20
OH*
+ hv OH + H 2 0
OH
(R1)
(R4) (R5) (R6) (R7)
where OH? denotes rotationally and/or vibrationally excited OH in the ground electronic state (XQ) and OH* denotes the excited electronic state responsible for the fluorescence emission ( A T ) . Reaction R3 can be neglected except at very low HzO concentrations. Fluorescence quenching of OH by He or O3 should be uniniportantlO compared to reaction R7 under our experimental conditions. Vibrational and rotational equilibration of OH+ by He or O3 is neglected in this scheme, although it is surely important at low HzO concentrations. The rates of reactions R1 and R5 are each directly proportional to the laser power; this explains the second-order dependency of the fluorescence signal on the laser power. The detailed vibronic energy distribution of OH is not considered in thiis scheme. The symbol OHt stands for a collection of vibronic states so that obviously reaction R4 does not occur in a single step; in fact, a small fraction of the OH produced by reaction R2 might already be in the vibronic state which is sampled by the laser (reaction R5). Nevertheless, thle simple R1-R7 mechanism presented above explains the experimental trends described earlier.
824
Molina et al.
The Journal of Physical Chemistry, Vol. 84, No. 8, 1980
TABLE 111: Ozone Photolysis Parameters Employed in the Calculations h , nm
307 308 309
U(O3), cm2 molecule-' 15.2 13.2
12.0
@(O('D)) 0.941 0.855 0.715
/ -
-A4
-I U 0 I n
I
3 Y
E
0
I 5
0 c
0
2 z
Lo
Ga
0 cl"
I-
5 2
I
V
z 0 V
0
T I M E , p SEC
Flgure 5. Computer simulation of the laser photolysis of a mixture of 0.02 torr of 03, 1 torr of H20, and 50 torr of He.
We have simulated this mechanism by using the rate constant values given in Table I1 and the ozone photolysis parameters presented in Table 111; the laser intensity profile was determined experimentally. The computer simulation was carried out with the CHEMK program developed by Whitten and Hogo.ll The actual OH-absorption cross section at the laser wavelength-which together with the laser intensity determines the rate of R5-cannot be estimated reliably under our experimental conditions. However, only a relative value for the fluorescence emission signal is needed, and the results of the simulation are not affected by the magnitude of the OH-absorption cross section as long as the computed [OH*] is negligible in comparison with [OH]. Figure 5 shows the results of a simulation for a typical set of conditions. The only adjustable parameter was k4, which is the effective rate constant for thermal equilibration of OH+by H20. A value of -6 X cm3 molecule-' s-l for this constant gives reasonable agreement with experiment, as shown in Figure 3, which displays the behavior of the fluorescence signals with increasing water vapor concentration. (The calculated and the measured signals have been normalized in this figure to unity, at [H20] = 1 torr.) We conclude that the chemical reaction R2 occurs almost an order of magnitude faster than rotational and/or vibrational equilibration (reaction R4) in our system; O('D) reacts with water practically on every cm3molecule-l s?), whereas on the collision (k2 = 2 X average -10 collisions might be required for thermal equilibration of OH. There is some indication12 that a fraction of the OH produced in reaction R2 is indeed vibrationally excited, but there is insufficient information on the detailed vibronic energy distribution for this reaction-as well as on the relaxation rates of OH+with H 2 0 as a collision partner-to make a more accurate simulation of the 03-H20 photolytic system. However, the primary purpose of using this system in the present work is to provide a calibration signal for OH in order to esti-
mate the HOCl photodissociation rate, and the simple reaction scheme described above is certainly sufficient for this purpose. If a large excess of H 2 0 is employed, i.e., [H20] 10 torr, reactions R2 and R4 go essentially to completionthat is, each O(lD) generates two thermalized OH radicals-on a time scale shorter than the laser pulse duration. In this case the integrated fluorescence emission signal ( S )can be approximated as in eq 1, where a(03)is
S = 2Cd03)dO(1D))[031/ [HzOl (1) the absorption cross section, +(O(lD))is the quantum yield for O('D) production at the laser wavelength, and C is a proportionality constant that depends on the responsivity of the photomultiplier, the geometry of the system, the spectroscopic properties of OH, and the energy and wavelength of the laser pulse. (All samples were optically thin, i.e., absorbance
